Partially-constrained thermomechanical fatigue of eutectic tin-bismuth/copper solder joints

Small bimetallic load-frames with reference assembly stiffness, k′, and fully-constrained shear strain, γ_fc, were used to simulate the thermo-mechanical conditions experienced by eutectic Bi-42wt.%Sn-to-Cu solder joints. Shear stress and strain were induced in the solder joint by a 45-minute, 0 to 100°C temperature cycle and were calculated from the assembly temperature, joint configuration, and measured elastic strain in the load-frame. Early in cycling, a hysteresis loop representing the maximum stress range and minimum strain range was reached. As damage accumulated in the solder, the stress range decreased and the strain range increased. The TMF life of the joints, defined by the load range drop, Φ, as a function of k′ and γ_fc, can be determined, defining an effective plastic strain range which allows data for various stiffnesses and thermal expansion mismatches to be summarized on a single Coffin-Mansion plot. The effective plastic strain range also provides an important link to conventional low cycle fatigue (LCF) data taken from an infinitely stiff load-frame.     

A microstructural study of the thermal fatigue failures of 60sn-40Pb solder joints

When an electronic package encounters thermal fluctuations, cyclical shear strain is imposed on the solder joint interconnections. The thermal cycling leads to a condition of thermal fatigue and eventual solder joint failure. This study was performed in order to understand the microstructural mechanisms that lead to solder joint failures in thermal fatigue. Thermal cycling tests were performed on 60Sn-40Pb joints using a -55° C to 125° C cycle and 19% imposed shear strain. A heterogeneously coarsened region of both Pb and Sn-rich phases develops within the 60Sn-40Pb solder joints. Cracks initiate in the heterogeneously coarsened Sn-rich phase at the Sn-Sn grain boundaries. Heterogeneous coarsening and failure occurs in both high (35 to 125° C) and low (-55 to 35° C) thermal cycles. The elevated temperature portion of the thermal cycle was found to be the most significant factor in the heterogeneous coarsening and failure of the solder joints.     

Influence of microstructure on fatigue crack growth behavior of Sn-Ag solder interfaces

The relationship between microstructure and fatigue crack growth behavior was examined at Sn-Ag solder interfaces on copper and electroless-nickel metallizations. On copper metallization, the solder interface was lined with a coarse Ag₃Sn intermetallic phase in addition to the Cu₆Sn₅ intermetallic and the adjacent solder alloy contained nodular Ag₃Sn phase. This interfacial microstructure was shown to result in inferior fatigue resistance, with the fatigue crack path following the interfacial Ag₃Sn intermetallic phase. In contrast, the solder interface on the electroless-nickel metallization was covered with a thin layer of Ni₃Sn₄ intermetallic phase, and the solder microstructure was composed of fine needles of Ag₃Sn phase dispersed in the Sn-rich matrix. This solder interface was found to be significantly more resistant to fatigue than the copper/Sn-Ag solder interface.     

Effect of cooling rate on microstructure and mechanical properties of eutectic Sn-Ag solder joints with and without intentionally incorporated Cu6Sn5 reinforcements

Solidification of eutectic Sn-Ag solder, with and without Cu₆Sn₅ composite reinforcements, on copper substrates, was investigated at two different cooling rates. The size, orientation, randomness, and overall morphology of the dendritic microstructure were examined as a function of cooling rate. Cu₆Sn₅ particle reinforcements were found to act as nucleation sites for dendrites, in addition to sites on the substrate/solder interface. The mechanical properties of these solders were also examined as a function of cooling rate. Solder joints with a lower load-carrying area were found to exhibit higher shear strength, but reduced ductility when compared to solder joints with more load carrying area.     

Phase coarsening and crack growth rate during thermo-mechanical cycling of 63Sn37Pb solder joints

The effects of phase coarsening in 63Sn37Pb solder joints produced by isothermal annealing and during thermo-mechanical cycling (TMC) on the fatigue cracked area growth rate dA_c/dN were investigated. The phase coarsening by isothermal annealing was in accord with the mechanism of interphase boundary diffusion; that during TMC was significantly greater than by isothermal annealing. The phase coarsening in the range considered (D=1–5 μm, N<150 cycles) had only little, if any, influence on dA_c/dN, which occurred mainly in Stage I of the fatigue crack growth rate regime. Results obtained previously, however, indicate that for N>150 cycles dA_c/dN is significantly decreased by a reduction in the as-reflowed phase size. Reasonable agreement occurred between the calculated fatigue lifetime vs initial as-reflowed phase size and that measured.     

Effects of cooling rate on mechanical properties of near-eutectic tin-lead solder joints

This paper reports the results of a study on the effect of the cooling rate during solidification on the shear creep and low cycle shear fatigue behavior of 60 Sn/40 Pb solder joints, and on bulk solder tensile properties. Solder joints were made with three different initial microstructures by quenching, air-cooling and furnace-cooling. They have similar steady-state strain rates under creep at relatively high shear stresses (i.e. in the matrix creep region) but creep at quite different strain rates at lower shear stresses (i.e. in the grain boundary creep region). These results are ascribed to the refined grain size and less lamellar phase morphology that results on increasing the cooling rate. Tensile tests on bulk solders that were cold-worked, quenched and furnace-cooled show that a faster cooling rate decreases the ultimate strength and increases the ductility at low strain rates. The fatigue life of quenched solder joints is shown to be longer than that of the furnace-cooled joints.     

The effect of aging on microstructure,room temperature deformation,and fracture of Sn-Bi/Cu solder joints

The effects of isothermal aging on the microstructure and mechanical behavior of Sn-Bi/Cu solder joints are reported. Lap shear solder joints of eutectic Sn-Bi solder were aged for 3 to 30 days at 80°C and then loaded to failure in shear. Changes in the joint microstructure including interphase coarsening, intermetallic growth, and evolution of the intermetallic/solder interface are documented. The aging experiments reveal the segregation of the Bi-rich phase of the solder to the intermetallic/solder interface. The ultimate shear strength and ductility of the joints are reported at strain rates of 4.0 × 10⁻¹ to 4.0 × 10⁻⁵ S⁻¹ for 3 and 30 days aging. The strength of the joints decreases with strain rate for both aging conditions; the ductility is low and independent of strain rate for the joints aged three days and increases considerably with reduced strain rate for joints, aged 30 days. Fractographs and cross sections of the failed joints detail the effect of aging on the fracture mechanism.     

Effects of prestrain,rate of prestrain,and temperature on the stress-relaxation behavior of eutectic Sn-3.5Ag solder joints

Stress-relaxation studies on eutectic Sn-Ag solder (Sn-3.5Ag in wt.%) joints were carried out at various temperatures after imposing different amounts and rates of simple shear strain. Stress-relaxation parameters were evaluated by subjecting geometrically realistic solder joints with a nominal joint thickness of ∼100 μm and a 1 mm × 1 mm solder-joint area. The peak shear stress during preloading and residual shear stress resulting from stress relaxation were higher at the low-temperature extremes than those at high-temperature extremes. Also, those values increased with increasing simple shear strain and the rate of simple shear strain imposed prior to the stress-relaxation events. The relaxation stress is insensitive to simple shear strain at 150°C, but at lower temperatures, a faster rate of simple shear strain causes a higher relaxed-stress value. The resulting deformation structures observed from the solder-joint side surfaces were also strongly affected by these parameters. At high temperature, grain-boundary sliding effects were commonly observed. At low temperature, intense shear bands dominated, and no grain-boundary sliding effects were observed.     

无铅焊点在热疲劳和电迁移作用下的组织演变

研究了Cu/Sn-58Bi/Cu焊点接头在室温和55℃下通电过程中阴极和阳极界面处微观组织的演变,电流密度均采用104A/cm2。结果表明,室温条件下通电达到25 d,Bi原子由阴极向阳极发生了扩散迁移,在阳极界面处形成了厚度约22.4μm的均匀Bi层,而阴极出现了Sn的聚集。加载55℃通电2 d后,焊点发生了熔融,阴极界面处形成了厚度为34.3μm的扇贝状IMC,而阳极界面IMC的厚度仅为9.7μm。在IMC层和钎料基体之间形成了厚度约7.5μm的Bi层,它的形成阻碍了Sn原子向阳极界面的扩散迁移,进而阻碍了阳极界面IMC的生长,导致了异常极化效应的出现。     

Finite element modeling of thermal fatigue and damage of solder joints in a ceramic ball grid array package

A nonlinear finite element model is presented for analyzing the cyclic and thermal fatigue loading and for viscoplastic damage characterization of the lead-tin (Pb-Sn) solder joints in a ceramic ball grid array (CBGA) surface mount package. An approach using a Δ ∈ _eq ⁱⁿ -modified Coffin-Manson equation is proposed to estimate the fatigue life of the solder joints. The Δ ∈ _eq ⁱⁿ represents a saturated equivalent inelastic strain range as determined by the finite element model. The present study shows that the predictied fatigue life and the associated damage mechanism of the solder joint agree reasonably well with the test data for the 18,25, and 32 mm CBGA packages run at a cyclic temperature load of 0°C/100°C with a frequency of 1.5 cycles per hour. Analysis also shows that a preferred failure site is expected to occur in and around the Pb37-Sn63 solder attachment of the solder joint. A time-dependent (creep induced) damage mechanism is found to be more pronounced than the time-independent (plastic deformation) mechanism.     

激光点状合金化对42CrMo钢疲劳裂纹扩展速率的影响

裂纹倾向严重和搭接区软化是激光表面强化技术面临的两大难题,为此,提出了一种激光点状合金化方法:使预涂在基体表面的合金涂层或由喷嘴送入的合金粉,在激光小孔效应作用下,与基体形成混合熔池,得到合金化点。研究表明,大量规则分布、互不重叠的合金化区及其周围的相变硬化区从总体上降低了疲劳裂纹扩展速率,提高了材料的疲劳寿命。对疲劳裂纹断口的初步分析显示,相变硬化区对阻滞裂纹扩展其到了重要作用。     

The growth of intermetallic compounds at Sn-Ag-Cu solder/Cu and Sn-Ag-Cu solder/Ni interfaces and the associated evolution of the solder microstructure

The evolution of intermetallics at and near SnAgCu/Cu and SnAgCu/Ni interfaces was examined, and compared to the behavior, near PbSn/metal and Sn/metal interfaces. Two different solder compositions were considered, Sn_93.6Ag_4.7Cu_1.7 and Sn_95.5Ag_3.5Cu_1.0 (Sn_91.8Ag_5.1 Cu_3.1 and Sn_94.35Ag_3.8Cu_1.85 in atomic percent). In both cases, phase formation and growth at interfaces with Cu were very similar to those commonly observed for eutectic SnPb solder. However, the evolution of intermetallics at SnAgCu/Ni interfaces proved much more complex. The presence of the Cu in the solder dramatically altered the phase selectivity at the solder/Ni interface and affected the growth kinetics of intermetallics. As long as sufficient Cu was available, it would combine with Ni and Sn to form (Cu,Ni)₆)Sn₅ which grew instead of the Ni₃Sn₄ usually observed in PbSn/Ni and Sn/Ni diffusion couples. This growing phase would, however, eventually consume essentially all of the available Cu in the solder. Because the mechanical properties of Sn-Ag-Cu alloys, depend upon the Cu content, this consumption can be expected to alter the mechanical properties of these Pb-free solderjoints. After depletion of the Cu from the solder, further annealing then gradually transformed the (Cu,Ni)₆Sn₅ phase into a (Ni,Cu)₃Sn₄ phase.     

Study on the microstructure of a novel lead-free solder alloy SnAgCu-RE and its soldered joints

This paper focused on the microstructure of SnAgCu-rare earth (RE) solder alloy and its small single-lap joints, focusing on phases present and the distribution of RE in the SnAgCu solder. Energy dispersive x-ray (EDX) analysis was used to observed the RE-rich phase. The RE atoms also tended to aggregate at boundaries of primary dendrites in the joints and form as a weblike structure, which surrounded the dendrites and restrained the dendrites from sliding or moving. It is assumed that this would strengthen the boundaries and increase the resistance to creep deformation of the solder matrix. The creep-rupture life of joints can be remarkably increased, at least seven times more than that of SnAgCu at room temperature. The aggregation mechanism of RE at dendrite boundaries in SnAgCu solder joints was presented. The drive for RE atoms to aggregate at the boundary is the difference of the lattice-aberration energy between the interior and the boundaries of the dendrites, which is caused by a solution of RE atoms.     

Effect of adding Sb on microstructure and adhesive strength of Sn-Ag solder joints

This study investigates the influence of adding Sb on the microstructure and adhesive strength of the Sn3.5Ag solder. Both solidus and liquidus temperatures increase as Sb additions increase. Adding 1.5wt.%Sb leads to the narrowest range (6.6°C) between the solidus and liquidus temperature of the solder. Adding Sb decomposes the as-soldered ringlike microstructure of Sn3.5Ag and causes solid-solution hardening. The as-soldered hardness increases with increasing Sb addition. For long-term storage, adding Sb reduces the size of the rodlike Ag₃Sn compounds. The hardness also increases with increasing Sb addition. Adding Sb depresses the growth rate of interfacial intermetallic compounds (IMCs) layers, but the difference between 1% and 2% Sb is not distinct. For mechanical concern, adding Sb improves both adhesive strength and thermal resistance of Sn3.5Ag, where 1.5% Sb has the best result. However, adding Sb causes a variation in adhesive strength during thermal storage. The more Sb is added, the higher the variation reveals, and the shorter the storage time requires. This strength variation helps the solder joints to resist thermal storage.     

热时效和焊点尺寸对SnAgCu微焊点强度的影响

针对高度为100～300μm的无铅钎料Sn-3.0Ag-0.5Cu微焊点,研究了等温热时效和焊点尺寸对其在100℃下拉伸强度的影响。结果表明,保持焊点直径不变时,高度为100,200和300μm微焊点未经热时效的平均拉伸强度分别为53.75,46.59和44.38MPa;热时效时间延长使微焊点内钎料合金显微组织明显粗化,导致焊点拉伸强度降低,前述三种高度的微焊点96h热时效后平均拉伸强度分别为44.13,38.38和33.48MPa,但96h热时效对IMC厚度无明显影响。     

Elevated temperature aging of solder joints based on Sn-Ag-Cu: Effects on joint microstructure and shear strength

The shear strength behavior and microstructural effects after aging for 100 h and 1,000 h at 150°C are reported for near-eutectic Sn-Ag-Cu (SAC) solder joints (joining to Cu) made from Sn-3.5Ag (wt.%) and a set of SAC alloys (including Co- and Fe-modified SAC alloys). All joints in the as-soldered and 100-h aged condition experienced shear failure in a ductile manner by either uniform shear of the solder matrix (in the strongest solders) or by a more localized shear of the solder matrix adjacent to the Cu₆Sn₅ interfacial layer, consistent with other observations. After 1,000 h of aging, a level of embrittlement of the Cu₃Sn/Cu interface can be detected in some solder joints made with all of the SAC alloys and with Sn-3.5Ag, which can lead to partial debonding during shear testing. However, only ductile failure was observed in all solder joints made from the Co- and Fe-modified SAC alloys after aging for 1,000 h. Thus, the strategy of modifying a strong (high Cu content) SAC solder alloy with a substitutional alloy addition for Cu seems to be effective for producing a solder joint that retains both strength and ductility for extended isothermal aging at high temperatures.     

Effects of strain ratio and tensile hold time on low-cycle fatigue of lead-free Sn-3.5Ag-0.5Cu solder

Low-cycle fatigue (LCF) behavior of a lead-free Sn-3.5Ag-0.5Cu solder alloy was investigated at various combinations of strain ratio (R = −1, 0, and 0.5) and tensile hold time (0, 10, and 100 sec). Results showed that the LCF life of the given solder, at each given combination of testing conditions, could be individually described by a Coffin-Manson relationship. An increase of strain ratio from R=−1 to 0 and to 0.5 would cause a significant reduction of LCF life due to a mean strain effect instead of mean stress effect. LCF life was also markedly reduced when the hold time at tensile peak strain was increased from 0 to 100 sec, as a result of additional creep damage generated during LCF loading. With consideration of the effects of strain ratio and tensile hold time, a unified LCF lifetime model was proposed and did an excellent job in describing the LCF lives for all given testing conditions.     

The effect of soldering process variables on the microstructure and mechanical properties of eutectic Sn-Ag/Cu solder joints

Fundamental understanding of the relationship among process, microstructure, and mechanical properties is essential to solder alloy design, soldering process development, and joint reliability prediction and optimization. This research focused on the process-structure-property relationship in eutectic Sn-Ag/Cu solder joints. As a Pb-free alternative, eutectic Sn-Ag solder offers enhanced mechanical properties, good wettability on Cu and Cu alloys, and the potential for a broader range of application compared to eutectic Sn-Pb solder. The relationship between soldering process parameters (soldering temperature, reflow time, and cooling rate) and joint microstructure was studied systemati-cally. Microhardness, tensile shear strength, and shear creep strength were measured and the relationship between the joint microstructures and mechani-cal properties was determined. Based on these results, low soldering tempera-tures, fast cooling rates, and short reflow times are suggested for producing joints with the best shear strength, ductility, and creep resistance.     

银含量对随机振动条件下无铅焊点可靠性的影响

为了探究银含量对无铅焊点在随机振动条件下的可靠性的影响,对Sn-3.0Ag-0.5Cu、Sn-1.0Ag-0.5Cu和Sn-0.3Ag-0.7Cu三种不同Ag含量材料的焊点做窄带范围内的随机振动疲劳实验,并对失效焊点进行分析。结果表明:三种材料焊点的失效位置基本都在靠近PCB侧,最外围焊点最容易失效,失效模式均为脆性断裂,并且随着Ag含量的降低,金属间化合物的厚度逐渐减小,焊点的疲劳寿命逐渐延长。     

Effects of cooling rate on the microstructure and tensile behavior of a Sn-3.5wt.%Ag solder

The tensile behavior and microstructure of bulk, Sn-3.5Ag solders as a function of cooling rate were studied. Cooling rate is an important processing parameter that affects the microstructure of the solder and, therefore, significantly influences mechanical behavior. Controlled cooling rates were obtained by cooling specimens in different media: water, air, and furnace. Cooling rate significantly affected secondary dendrite-arm size and spacing of the Sn-rich phase, as well as the aspect ratio of Ag₃Sn. The Sn-rich dendrite-arm size and spacing were smaller for water-cooled specimens than for air-cooled specimens. Furnace cooling yielded a nearly eutectic microstructure because the cooling rate approached equilibrium cooling. The morphology of Ag₃Sn also changed from spherical, at a fast cooling rate, to a needlelike morphology for slower cooling. The changes in the microstructure induced by the cooling rate significantly affected the mechanical behavior of the solder. Yield strength was found to increase with increasing cooling rate, although ultimate tensile strength and strain-to-failure seemed unaffected by cooling rate. Cooling rate did not seem to affect Young’s modulus, although a clear coorelation between modulus and porosity was obtained. The mechanical behavior was correlated with the observed microstructure, and fractographic analysis was employed to elucidate the underlying damage mechanisms.